Stroke is a leading cause of long-term disability in the US and the third leading cause of death after heart disease and cancer. The identification of triggers of stroke onset is of significant public health interest. Many epidemiologic studies have found an association between short-term increases in particulate air pollution and cardiovascular morbidity and mortality. These effects are likely mediated by neural, hematologic, and inflammatory responses to inhaled particles. Exposure to particulate matter may also increase the risk of ischemic stroke. Recent studies using administrative databases have reported equivical results. Accordingly, the goal of this Project is to quantitatively evaluate the association between short-term increases in ambient particulate matter and the risk of acute ischemic stroke. Specifically, we will evaluate: 1) the association between short-term increases in particulate matter and the risk of acute ischemic stroke, 2) the time course of this effect, 3) the specific constituents of ambient particulate matter to which the effect is most likely attributable, and 4) variation in effect magnitude by etiological subtype of ischemic stroke. To accomplish these aims we will use the services and expertise of the PPG Cores and will apply the case-crossover methodology to the study of 1200 patients hospitalized with acute ischemic stroke. Even though the magnitude of any effect is expected to be small, given the large number of people simultaneously at risk for stroke and exposed to urban air pollution, the attributable risk may be quite significant. Clarifying the impact of particulate air pollution on stroke onset is important for regulatory agencies. Additionally, information about the time course of the effect and identification of important particle constituents may provide insight into the mechanisms of particulate-induced cardiovascular events.